Iron-nickel alloy with low thermal expansion coefficient and exceptional mechanical properties

ABSTRACT

The invention relates to creep-resistant iron-nickel alloys with a low thermal expansion coefficient that contain (in weight %) 0.008 to 0.12 C, 0.05 to 0.30% Mn, 0.05 to 0.30% Si, 0.2 to 0.9% Mo, 0.1 to 0.3% Cr, 0.03 to 0.15% Nb, a maximum content of 0.5% Co as well as a content of 36.0 to 36.5% Ni, and a balance of iron and production-related impurities. The alloys have a thermal expansion coefficient of less than 2.0×10 −6 /K in the temperature range of 20 to  100 ° C.

[0001] The present invention relates to an iron-nickel alloy with lowthermal expansion coefficient and exceptional mechanical properties.

[0002] It is a known fact that iron-based alloys with approximately 36%nickel have a low thermal expansion coefficient in the temperature rangefrom 20 to 100° C. These alloys have therefore been used for years whereconstant lengths are required even under varying temperatures such as inprecision instruments, watches or bi-metals. With the development of thecolor television sets and computer monitors towards ever greaterresolution, color fidelity and contrast, even under unfavorable lightingconditions, and in particular in view of the trend toward ever flatterand larger screens, iron-nickel materials are used more and more forshadow masks.

[0003] Technical iron-nickel alloys with approximately 36% nickel have athermal expansion coefficient between 1.2 and 1.8×10⁻⁶/K, in thesoft-annealed state as indicated in the “Stahl-Eisen Werkstoffblatt”(steel-iron materials journal) (SEW-385, Edition 1991) in thetemperature range from 20 to 100° C. such prevailing in conventionalscreen tubes. Further developed materials with approximately 36% nickelare also being used in particular for shadow masks and attain lowthermal expansion coefficients between 0.6 and 1.2×10⁻⁶/K in thetemperature range from 20 to 100° C.

[0004] For shadow masks pre-stretched in frames, a low-expansionmaterial with better creep resistance than that of alloys used so far isdemanded. The shadow masks and the frame parts for the shadow masks aresubjected to a so-called black-annealing process at temperatures up toapproximately 580° C. Thereby a dark iron oxide layer is produced withwhich a better visual picture quality is achieved.

[0005] The iron-based alloy with approximately 36% nickel used until nowachieves a creep resistance A₈₀ of approximately 2.6% under thefollowing test conditions: 1 hour at 580° C. with a load of 138 MPa.

[0006] The shadow masks are pre-stressed in vertical direction by meansof the vertical frame parts. Until now iron-nickel alloys withapproximately 41% nickel were used, these alloys being known asmaterials for melting metal to glass for example, or for lead frames.The technological characteristics are as follows: The creep resistanceA₈₀ is approximately 0.5% measured under the same testing conditions asdescribed previously for the alloy containing 36% nickel, i.e. 1 hour at580° C under a load of 138 MPa. The vertical frame parts made of thisalloy become more elongated according to a thermal expansion coefficientof approximately 4.8×10⁻⁶/K within a temperature range from 20 to 100°C. than the shadow mask made of the iron-nickel alloy with approximately36% nickel.

[0007] Newly developed materials for horizontal frame parts should havethe same low thermal expansion characteristics as the iron-nickel alloyused until now for shadow masks which, in addition to iron andapproximately 36% Ni, essentially only contain residues due tomanufacture in small amounts.

[0008] Just as for the shadow masks, materials with a better creepresistance at temperatures up to 580° C. than that of alloys used untilnow are demanded for the frame parts. The magnitude and thetemperature-dependent course of the expansion coefficients should benearly identical with those of the materials used until now.

[0009] It is furthermore known that suitable additives to iron-nickelalloys can increase hardening. For this e.g. molybdenum and chromium incombination with carbon can be considered. At the same time thepermanent elongation limit and strength increase.

[0010] However excessive totals of the elements molybdenum, chromium andcarbon can raise the thermal expansion coefficients excessively.

[0011] As a wire material for overland lines, an iron-nickel alloybecame known, containing approximately 38 Ni, 2% Mo, 0.8% Cr and 0.25% Cas well as impurities and iron residue resulting from production. In thesolution-annealed state these alloys have a thermal expansioncoefficient of approximately 4×10⁻⁶/K between 20 and 100° C. The tensilestrength can reach values above 1000 N/mm² in the hardened state.

[0012] JP-A 10060528 discloses an invar alloy with the followingcombination: ≦0.1% C, 0.35% Si, ≦10% Mn, 0.015% P, ≦0.005% S≦0.3% Cr,35-37% Ni, 0-0.5% V, 0.01% Al, 0-1% Nb, 0-0.005% B, ≦0.005% N, ironresidue as well as impurities resulting from product ion.

[0013] JP-A 10017997 describes another invar alloy that is composed asfollows (in percentage by weight): 0.015%-0.10% C, ≦0.35% Si, ≦1.0% Mn,≦0.015% P, ≦0.0010% S, 0.3% Cr, 35-37% Ni, 0-0.5% Mo, 0-0.05% V, ≦0,01Al, 0.15-<1.0% Nb, ≦0.003% Ti, ≦0.005% N, with S being set to ≦0.002%and Ti to 0.05-0.2%. In addition 0.0005%-0.005% B can be added to thealloy. The remainder is iron and impurities resulting from production.

[0014] JP-61183443 relates to an alloy with low thermal expansioncoefficients, containing the following alloy elements: 25-50% Ni, ≦0.30%C, ≦2.0% Si+Mn, ≦10% of one or several of the elements Al, Cr, Mo, W, V,Nb, Ta, Ti, Zr and Hr, Fe residue as well as impurities resulting fromproduction. This alloy is to be used among other things for parts ofelectronic tubes, measuring instruments or similar devices.

[0015] Finally DE-A 3642205 describes a shadow mask material whichconsists essentially, in percentage by weight, of ≦0.10% C, ≦0.30% Si,≦≦0.30% Al, 0.1-1.0% Mn, 34-38% Ni, one or several additional elementssuch as Ti, Zr, B, Mo, Nb, N, P, Cu, V, Mg, Co and W in contents from0.01 to 1.0%, iron residue and unavoidable impurities.

[0016] It is the object of the present invention to develop acreep-resistant iron-nickel alloy with low thermal expansion so that itmay no longer have the disadvantages indicated in the state of the art,may be economical to produce and may be used in many different technicalapplications.

[0017] This object is attained with a creep-resistant and low-expansioniron-nickel alloy containing (in percentage by weight), in addition to0.008 to 0.12% C, 0.05 to 0.30% Mn and 0.05 to 0.30% Si and 0.2 to 0.9%Mo and 0.1 to 0.3% Cr and 0.03 to 0.15% Nb and max. 0.5% Co as well asfrom 36.0 to 36.5% Ni, the remainder being iron and impurities resultingfrom production, whereby the alloy has a thermal expansioncoefficient<2.0×10⁻⁶/K within a temperature range from 20 to 100° C.

[0018] A preferred alloy contains (in percentage by weight)

[0019] 0.08 to 0.11% C

[0020] 0.15 to 0.25% Cr

[0021] 0.10 to 0.20% Mn

[0022] 0.10 to 0.15% Si

[0023] 0.5 to 0.7% Mo

[0024] 0.05 to 0.09% Nb

[0025] max. 0.1% Co

[0026] 36.0 to 36.5% Ni

[0027] remainder being iron and impurities resulting from production.

[0028] If necessary further elements can be contained by the inventivealloy (in percentage by weight)

[0029] max. 0.002% S

[0030] max. 0.01% Ti

[0031] max. 0.2% Cu

[0032] max. 0.010% P

[0033] max. 0.01% Al

[0034] max. 0.003% Mg.

[0035] Several analysis of the inventive alloy have shown that it ispossible to realise a thermal expansion coefficient<1.6×10⁻⁶/K in thetemperature range from 20 to 100° C.

[0036] The required technological properties for utilization as amaterial, in particular for vertical frame parts of shadow masks, can beselected by means of the iron-nickel alloy according to the invention,whereby the composition with regard to Ni, Mo, Cr and C contents can beselected so that the desired thermal expansion coefficients andmechanical properties are obtained.

[0037] The object of the invention can be used advantageously for thefollowing objects, in addition to frame parts and shadow masks forscreens and monitors:

[0038] passive components of thermo-bimetals

[0039] components in laser technology

[0040] lead frames

[0041] components of electron guns, in particular TV tubes

[0042] components for the production, storage and transportation ofliquefied gas

[0043] Preferred alloys according to the invention are compared belowwith alloys according to the state of the art with respect to theirmechanical properties.

[0044] A preferred composition E1 of the alloy according to theinvention for utilization as a material for the vertical frame, e.g. fora screen, contains (in percentage by weight) in addition to 36 to 36.5%Ni, 0.2 to 0.9% Mo, <0.2% Cr and in addition 0.08 to 0.12% C, also max.0.3% Mn, max. 0.3% Si, max. 0.15% Nb and the usual impurities in verysmall quantities resulting from the production process. The thermalexpansion coefficient of approximately 2.0×10⁻⁶/K between 20 and 100° C.and the overall temperature-dependent evolution of the expansioncoefficients between room temperature and 600° C. are comparable withthe value or the evolution of the two-material alloy T2 used until nowaccording to the state of the art which, in addition to iron andapproximately 36% Ni, only contains impurities resulting from theproduction process. The improvement of the mechanical propertiesnecessary for the application is achieved with the alloy according tothe invention, in particular insofar as the creep-resistance, measuredon a 1.4 mm thick cold-rolled sample as extension A80 at 580° C. andthis even with a greater load of 200 MPa for one hour, in other wordsapproximately 0.02%. The alloy E1 according to the invention excelsthrough its outstanding workability and requires no additional processsteps in production. This means that to obtain the especially goodmechanical characteristics, no additional hardening/heating treatment isneeded, such as would be needed for example with γ′-precipitationhardenable alloys. The frame can be bent into shape directly in thecold-rolled state. The mechanical properties as described above arepresent in this state. In addition the long-term stability of itsthermal properties meets all requirements.

[0045] Another preferred composition E2 of the alloy according to theinvention for application as material for a vertical frame, e.g. of amonitor, contains (in % by weight) in addition to 36 to 36.5% Ni, 0.4 to0.8% Mo, 0.1 to 0.3% Cr and in addition 0.08 to 0.12% C as well as max.0.3% Mn, max. 0.3% Si, max. 0.15% Nb and the usual impurities resultingfrom the production process in only very small amounts. The thermalexpansion coefficient between 20 and 100° C. at approximately 1.8×10⁻⁶/Kis lower than in the case of the alloy E1 according to the invention.The improvement of the mechanical properties required for theapplication is also attained with the alloy E2 according to theinvention, in particular insofar as the creep-resistance, measured asextension A80 at 580° C. on a cold-rolled sample of 1.4 mm thickness isexpressed by a value of approximately 0.03%, even with a load of 200 MPaduring one hour. The alloy E2 according to the invention is alsoremarkable through its outstanding workability and requires noadditional process steps in production. This means that in order toobtain the especially good mechanical properties, no additionalhardening/heating treatment is needed. The frame can be bent into shapedirectly in the cold-rolled state. The mechanical properties asdescribed above are present in this state. In addition the long-termstability of its thermal properties meets all requirements.

[0046] The required technological properties for utilization as amaterial, in particular for vertical frame parts for shadow masks can beobtained with the iron-nickel alloy according to the invention, wherebythe composition with regard to Ni Mo, Cr, Nb and C contents can beselected so that the desired thermal expansion coefficients andmechanical properties are present.

[0047] The mechanical properties that were determined in a hot-drawingtest without load at the testing temperature of 580° C. as well as themagnetic coercitive field force as well as the thermal expansioncoefficients are shown in Table 1 for the alloys E1 and E2 according tothe invention as compared with the properties of the alloys T1 and T2 ofthe state of the art. Element E1 E2 T1 T2 Hot drawing test at 580° C.Permanent elongation limit Rp_(0.005) (N/mm²) 189 261 Rp_(0.1) (N/mm²323 347 Rp_(0.2) (N/mm² 460 438 312 322 Tensile strength 485 451 411 381R_(m), (N/mm²) Elongation at rupture A₈₀ (%) 3.80 4.40 7.5 6.4 Creepresistance 200N/mm² 138 N/mm² load 200 N/mm² 0.02 0.03 0.54 2.61

[0048] Thermal expansion coefficients (from 20° C. to testingtemperature T in 10 6/K) T(° C.) 100 2.0 1.8 4.88 1.26 200 2.6 2.6 4.492.45 300 5.2 5.3 4.52 5.47 400 8.1 8.2 6.31 8.28 500 10.0 10.2 8.1410.23 600 11.4 11.6 9.69 11.61

[0049] Table 1: Mechanical characteristics permanent elongation limit,tensile strength, elongation to rupture at 580° C., determined in hotdrawing test, as well as creep resistance for 1 hour at 580° C. with aload of 138 MPa or 200 MPa and the thermal expansion coefficients of thealloys E1 and E2 according to the invention as compared with the alloysT1 and T2 of the state of the art. The testing sample was made from a1.4 mm cold-rolled band.

[0050] Examples of the chemical composition of the alloys E1 and E2according to the invention as compared with the composition of thealloys T1 and T2 of the state of the art are listed in Table 2. Element(% by mass) E1 E2 T1 T2 C 0.108 0.109 0.007 0.003 S 0.0008 0.0008 0.0030.0003 N 0.001 0.001 0.002 0.0025 Cr 0.015 0.20 0.03 0.03 Ni 36.40 36.4040.80 36.15. Mn 0.13 0.14 0.55 0.24 Si 0.10 0.10 0.17 0.06 Mo 0.70 0.620.01 0.05 Ti 0.01 0.01 0.005 <0.01 Nb 0.05 0.05 <0.01 0.01 Cu 0.01 0.010.04 0.05 Fe R e m a i n d e r P 0.002 0.002 0.003 0.002 Al 0.005 0.0010.002 0.007 Mg 0.001 <0.001 <0.001 0.002 Co 0.01 0.01 0.04 0.04 O 0.00550.0055 0.002 0.002

[0051] Table 2: Examples of chemical compositions of the alloys E1 andE2 according to the invention as compared with examples of compositionsof alloys T1 and T2 of the state of the art.

[0052] For certain fields of application it may be advisable to addcobalt in the indicated amounts (in % by weight) of the alloy accordingto the invention. Preferred additions of cobalt (in % by weight) arebetween 0.1 and 0.5%0, whereby the nickel content must then be adjustedcorrespondingly.

1. Creep-resistant iron-nickel alloy with low thermal expansion,containing (in % by weight) in addition to 0.008 to 0.12% C, 0.05 to0.30% Mn and 0.05 to 0.30% Si, also 0.2 to 0.9% Mo and 0.1 to 0.3% Crand 0.03 to 0.15% Nb and max. 0.5% Co as well as 36.0 to 36.5% Ni, theremainder being iron and impurities resulting from the productionprocess, whereby the alloy has a thermal expansioncoefficient<2.0×10⁻⁶/K within a temperature range from 20 to 100° C. 2.Alloy according to claim 1 containing (in % by weight) 0.08 to 0.11% C0.15 to 0.25% Cr 0.10 to 0.20% Mn 0.10 to 0.15% Si 0.5 to 0.7% Mo 0.05to 0.09% Nb max. 0.1% Co 36.0 to 36.5% Ni the remainder being iron andimpurities resulting from the production process, whereby the alloy hasa thermal expansion coefficient<2.0×10⁻⁶/K within the temperature rangefrom 20 to 100° C.
 3. Alloy according to claim 1 further containing (in% by weight) max. 0.002% S max. 0.01% Ti max. 0.2% Cu max. 0.010% P max.0.01% Al max. 0.003% Mg
 4. Ally according to claim 2 whereby the alloyhas a thermal expansion coefficient<1.6×10⁻⁶/K within the temperaturerange from 20 to 100° C.
 5. Shadow masks of screens and monitors made ofthe iron-nickel alloy of claim
 1. 6. Frame parts, in particular verticalframe parts, of shadow masks made of the iron-nickel alloy of claim 1.7. Passive components of thermal bi-metals made of the iron-nickel alloyof claim
 1. 8. Components for the production, storage and transportationof liquefied gas made of the iron-nickel alloy of claim
 1. 9. Componentsin the laser technology made of the iron-nickel alloy of claim
 1. 10.Lead frames made of the iron-nickel alloy of claim
 1. 11. Components forelectron guns, in particular for television tubes, made of theiron-nickel alloy of claim 1.